How do phase changes classify as physical changes?

Phase changes are classified as physical changes because they alter the state of a substance without changing its chemical composition.

In more detail, a phase change refers to the transition of matter from one state to another, such as from a solid to a liquid (melting), a liquid to a gas (evaporation), or a gas to a solid (deposition). These changes are considered physical because they only affect the form of the substance, not its chemical identity. The molecules of the substance remain the same before and after the phase change, only their arrangement and movement change.

For example, when ice melts into water, it undergoes a phase change from solid to liquid. The water molecules in the ice are arranged in a rigid, ordered pattern, but when the ice melts, the water molecules become less ordered and can move more freely. Despite this change in state, the water molecules themselves do not change. They are still H2O molecules, whether they are in the solid, liquid, or gas phase.

Similarly, when water evaporates into steam, it undergoes a phase change from liquid to gas. The water molecules in the liquid state are close together and move randomly, but when the water evaporates, the water molecules spread out and move more rapidly. Again, despite this change in state, the water molecules do not change. They remain as H2O molecules.

In contrast, a chemical change involves a change in the chemical composition of a substance. This means that the molecules of the substance are altered in some way, resulting in a new substance with different properties. For example, when hydrogen gas reacts with oxygen gas to form water, this is a chemical change because new molecules (H2O) are formed from the reaction of hydrogen (H2) and oxygen (O2) molecules.

Therefore, phase changes are classified as physical changes because they involve a change in the state of a substance, not a change in its chemical composition.